The present invention relates to a heat-shrinkable multilayer film suitable for use as a heat-shrinkable packaging material including an outer surface layer comprising a polyester resin, an intermediate layer comprising a polyamide resin and a sealable resin layer, and also a process for production of such a heat-shrinkable multilayer film.
Hitherto, it has been widely practiced to make a packaging material, such as a bag or a pouch, of a heat-shrinkable resin film by means of a bag-making machine, and fill the packaging material with a content material or subject the packaging material to flow packaging, tray packaging or skin packaging by means of an automatic packaging machine, principally for packaging of ham, sausage and other foods.
Such heat-shrinkable resin films have been conventionally provided in many cases in the form of heat-shrinkable multilayer films including various forms or structures of laminate of various resins inclusive of a polyolefin resin (hereinafter sometimes abbreviated as xe2x80x9cPOxe2x80x9d) excellent in sealability. extrudability and stretchability, a polyamide resin (abbreviated as xe2x80x9cPAxe2x80x9d) excellent in mechanical properties, stretchability and gas-barrier property, a vinylidene chloride resin (abbreviated as xe2x80x9cVdClxe2x80x9d) or an ethylene-vinyl alcohol copolymer resin (abbreviated as xe2x80x9cEVOHxe2x80x9d) that is particularly excellent in gas-barrier property, an ethylene-vinyl acetate copolymer resin (abbreviated as xe2x80x9cEVAxe2x80x9d) or a modified polyolefin resin (abbreviated as xe2x80x9cM-POxe2x80x9d) in a sense of including an ionomer resin that is excellent in heat-sealability and adhesiveness, etc. Representative examples of laminate form or structure may include: PO/PA/PO, EVA/PA/EVA, PO/PA/EVA (e.g., as disclosed in Japanese Laid-Open Patent Application (JP-A) 51-92882, JP-A 55-130743, JP-A 8-23035), and further PA/EVA/VdCl/EVA/PA (e.g., as disclosed in JP-A 62-273849) wherein respective layers are enumerated from the outer surface layer to the inner surface layer in each laminate structure.
In case where such a heat-shrinkable multilayer film having a representative laminate structure of PO/PA/PO, EVA/PA/EVA or PO/PA/EVA is subjected to heat-sealing by means of a sealing bar included in a representative automatic packaging mode, the PO or EVA layer constituting the inner surface layer shows good sealability but the outer surface layer of PO or EVA is also liable to adhere to the seal bar due to heat fusion, so that it is difficult to increase the heat-sealing speed, and accordingly the automatic packing speed. Further, a film having such a laminate structure is also generally liable to be insufficient in transparency. Further, a film having an outer surface layer of PO is liable to exhibit insufficient printability. Most of such difficulties are not encountered in a heat-shrinkable multilayer film having a laminate structure of PA/EVA/VdCl/EVA/PA, but this film is accompanied with a difficulty that its film properties are liable to be changed depending on changes in environmental conditions because the surface layers of PA are rich in moisture-absorptivity.
In order to solve many of the problems accompanying the laminate structures of conventional heat-shrinkable multilayer films, our research and development group has proposed a heat-shrinkable multilayer film including a relatively thin outer surface layer of a polyester resin (hereinafter sometimes, representatively abbreviated as xe2x80x9cPETxe2x80x9d) which had not used hitherto for laminate film formation by co-extrusion and stretching with a polyamide resin, and an intermediate layer of polyamide resin, realized by harmonizing the stretchability of the outer surface layer of PET with that of the intermediate layer of PA (JP-A 4-99621. corresponding to U.S. Pat. No. 5336549 and EP-A 476836). Representative example laminate structures of the heat-shrinkable multilayer film disclosed therein include; PET/PA/PO, PET/M-PO/PA/M-PO/PO and PET/M-PO/PA/EVOH/M-PO/PO. In the heat-shrinkable multilayer film, the outer surface layer of exhibits essentially larger heat resistance and better printability than that of PO as a typical sealable resin, and also better anti-moisture stability than that of PA. Further, as the co-stretchability of the outer surface layer of PET and the intermediate layer of PA has been harmonized, the resultant film after biaxial stretching exhibits a relatively good size stability without applying a heat-treatment which causes a lowering in heat-shrinkability, thus providing a heat-shrinkable multilayer film exhibiting excellent performances as a heat-shrinkable packaging material.
As a result of further study of ours, it has been however found that the above-mentioned PET-surfaced heat-shrinkable multilayer film causes several problems because of its inherent heat-shrinkability when applied to automatic packaging. The problems are, e.g., as follows. (a) In case of tray packaging wherein a processed meat product is placed on a foamed plastic tray and then wrapped with a heat-shrinkable film, followed by heat-shrinking of the film to provide a packaged product with the film intimately attached to the content, the tray in the packaged product is deformed. (b) In case of pizza packaging wherein a pizza is wrapped with a heat-shrinkable film in the form of sealing at three sides, followed by heat-shrinking to provide a packaged product, the pizza is bent to lower the merchandise value. (c) When a deep-drawn container or tray containing a content therein is covered with a heat-shrinkable film as a lidding material, followed by shrinking of the film to provide a packaged product, the container or tray is deformed. (d) When an angular product (such as ham) is wrapped with a heat-shrinkable film and subjected to a boiling treatment (e.g., at 90xc2x0 C. for 10 min.) for both heat-shrinking and sterilization, the angles of the product are rounded to lower the merchandise value. (e) In case of automatic packaging including heat-sealing by means of a sealing bar, a sealing failure occurs in some cases if the sealing speed is increased. Particularly, pinhole defects occur in some cases due to double sealing in case where the sealing interval is reduced. (f) Irregular shrinkage of the film occurs during printing in some cases, whereby the resultant printed film is accompanied with print deviation or non-flat rolled state, thus resulting in difficulties in bag-making and packaging thereafter.
Accordingly, a principal object of the present invention is to provide a PET-surfaced heat-shrinkable multilayer film adapted to automatic packaging.
As a result of our further study for accomplishing the above object, it has been found that most of the above-mentioned problems (a)-(f) accompanying the known PET-surfaced heat-shrinkable multilayer film are attributable to its inherent heat-shrinkability but, more than the degree or magnitude of the heat-shrinkability, are attributable to an excessively large stress occurring in the film during the heat-shrinkage, i.e., an excessively large heat-shrinkage stress. It has been also discovered that, while a heat-treatment or annealing is conventionally applied to a PET (polyester resin) film as a post-treatment after biaxial stretching for removing the heat-shrinkability to provide a size stability, a PET-surfaced multilayer film laminated with a PA layer after biaxial stretching under appropriate conditions may be subjected to an optimum heat-treatment (i.e., a heat-treatment capable of exerting a relaxation at a relatively low temperature) to effectively reduce the heat-shrinkage stress while retaining a necessary level of heat-shrinkability, thus solving most of the above-mentioned problems encountered in automatic packaging. It has been also found that in order to effect such a heat-treatment exerting a uniform relaxation effect at a low temperature after an inflation process as a preferable biaxial stretching process, it is extremely preferred to use steam or warm water having a large heat capacity as the heat-treatment medium.
The heat-shrinkable multilayer film according to the present invention is based on the above findings and more specifically comprises at least three layers including an outer surface layer (a) comprising a polyester resin, an intermediate layer (b) comprising a polyamide resin and an inner surface layer (c) comprising a sealable resin; said multilayer film exhibiting a heat-shrinkage stress at 50xc2x0 C. of at most 3 MPa both in longitudinal direction and transverse direction, and a hot water shrinkability at 90xc2x0 C. of at least 20%.
According to the present invention, there is also provided a process for producing a heat-shrinkable multilayer film, comprising the steps of:
co-extruding at least three species of melted thermoplastic resins to form a tubular product comprising at least three layers including an outer surface layer (a) comprising a polyester resin, an intermediate layer (b) comprising a polyamide resin and an inner surface layer (c) comprising a sealable resin,
cooling with water the tubular product to a temperature below a lowest one of the melting points of the polyester resin, the polyamide resin and the sealable resin constituting the layers (a), (b) and (c),
re-heating the tubular product to a temperature which is at most the lowest one of the melting points of the polyester resin, the polyamide resin and the sealable resin constituting the layers (a), (b) and (c),
vertically pulling the tubular product while introducing a fluid into the tubular product to stretch the tubular product at a ratio of 2.5-4 times both in a vertical direction and in a circumferential direction, thereby providing a biaxially stretched tubular film,
folding the tubular film,
again introducing a fluid into the folded tubular film to form a tubular film,
heat-treating the tubular film from its outer surface layer (a) with steam or warm water at 60-98xc2x0 C., and
cooling the heat-treated tubular film to provide a biaxially stretched film exhibiting a heat-shrinkage stress at 50xc2x0 C. of at most 3 MPa both in longitudinal direction and transverse direction, and a hot water shrinkable at 90xc2x0 C. of at least 20%.